This invention relates to a process for the stabilization of certain fluoropolymers, especially those useful in many high technology applications such, e.g., as electronic equipment and optical fibers.
Many fluoropolymers are known in the art. They include especially various copolymers of two or more comonomers such as, for example, tetrafluoroethylene (TFE), hexafluoropropene, chlorotrifluoroethylene, perfluoro(methyl vinyl ether), and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD).
Such copolymers often are melt-processible and thus can be fabricated at high temperatures. However, they frequently suffer some deterioration during high temperature processing and thus lose some of their desirable properties such as, for example, good optical clarity. This thermal deterioration can be traced to the presence of various labile end groups, such as, e.g., carboxyl (--COOH) and fluorocarbonyl (--COF). The former tends to eliminate carbon dioxide at high temperatures, while the latter, while more thermally stable, nevertheless tends to hydrolyze in the presence of moisture, which normally cannot be completely avoided, and is converted to carboxyl groups. Such hydrolysis also results in the the evolution of hydrofluoric acid which is corrosive to most materials of industrial importance, including many metals, glass and quartz.
The removal of unstable end groups has long been an important part of the technology of perfluorinated melt-processible copolymers of TFE. Schreyer, U.S. Pat. No. 3,085,083, treated such polymers "with water, preferably in the presence of inorganic compounds having a pH of at least 7, such as stable bases,--- at a temperature of 200-400.degree. C., and recovering a fluorocarbon polymer having at least half of all the end-groups in the form of difluoromethyl groups". However, polymers with difluoromethyl end groups, --CF2H, are inferior for use in optical fibers because of their absorption of light at certain wavelengths.
Buckmaster et al., U.S. Pat. No. 4,675,380, teach the fluorination of melt-processible TFE copolymers which have been coagulated by stirring in the presence of a mineral acid and a water-immiscible liquid and then isolated. The total number of unstable end groups was reduced to less than 80 per 10.sup.6 carbon atoms.
U.K. Pat. No. 1,210,794 to Du Pont discloses the fluorination of fluorocarbon copolymers to reduce the number of unstable end groups. When using the copolymers of interest in the present invention, the process of that patent employed a fluorination temperature of at least 225.degree. C. to remove all unstable end groups. The patent makes no specific mention of --COF end groups, which have since been found to be the most difficult groups to fluorinate.
Anderson et al., U.S. Pat. No. 4,594,399, discloses perfluoro(2-methyl-1,3-dioxole) and its copolymers.
Squire, U.S. Pat. No. 4,399,264, discloses perfluorodioxole and its copolymers.
Squire, U.S. Pat. No. 4,530,569, discloses amorphous copolymers of PDD and optical fibers clad with these copolymers.
Core/cladding optical fibers described in U.S. Pat. No. 4,530,569 have good heat resistance. Example 11 of that patent describes a fused silica optical fiber clad with an amorphous PDD/TFE copolymer, which had an optical attenuation of 113 dB/Km. While adequate for some uses, this is not adequate for long distance transmission of light signals. Normally, a loss of more than 20 dB/km is undesirable in some applications. Thus, while a fiber with attenuation of 20 dB/km can be used for distances up to about 1000 m, a fiber with attenuation of 113 dB/km would be useful for distances up to about 180 m, and a fiber with attenuation of 2000 dB/km would still be useful for distances up to about 10 m. Numerous applications for optical/electric switch sensors involve distances of 10 m or less, e.g., in microwave ovens, copy machines, chemical reactors, and furnaces, but lower attenuation makes an optical fiber more broadly useful.
Melt-processible copolymers containing multiple bonds, e.g., carbonyl groups, may be unsuitable for optical fibers, because multiple bonds absorb light of certain wavelengths. Multiple bonds in perhalogenated polymers can be destroyed by fluorination at or above 200.degree. C.
While fluorination of fluoropolymers reduces the concentration of multiple bonds and unstable end groups, complete fluorination requires high temperatures, usually above 200.degree. C., to remove substantially all --COF groups. However, when the polymers soften or begin melting at the fluorination temperature, such a process causes agglomeration of polymer particles, which leads to difficulties in their further handling and processing.
Furthermore, high temperature fluorination can cause equipment corrosion.
It is, therefore, desirable to make possible fluorination of fluoropolymers at temperatures not exceeding 200.degree. C., and in any event below temperatures at which the fluoropolymers melt or significantly soften.